The Bush administration's plan for a new set of nuclear warheads to replace the aging ones in the U.S. stockpile received a tough review today from some outside experts, who criticized the plan's cost.

The group included some top scientists and three former directors of laboratories that design nuclear weapons.

The U.S. nuclear arsenal includes nine types of warheads, the majority of which were designed more than 30 years ago. Some experts say its time to replace them with newer, simpler warheads that are more resistant to aging and cheaper to maintain.